(1) Field of the Invention
This invention relates to an abrasive molding and an abrasive disc provided with at least one abrasive molding, which are used in a process for polishing or chemicomechanically polishing substrate materials, for example, for substrates such as a semiconductor substrate, a chemical compound semiconductor substrate, a mild metal substrate, a glass substrate and silica glass substrate, optical materials, and organic materials such as plastic and other resin materials.
(2) Description of the Related Art
With the advance of industries including an optical industry and an electronic industry, a higher precision is required for processing materials for a magnetic disc, a semiconductor substrate, an optical material and other substrate materials. That is, there is an increasing demand for obtaining higher smoothness and flatness by polishing the material surface in the finishing process thereof.
A loose abrasive machining has been widely employed in the conventional polishing process, wherein a substrate material is polished with a polishing pad made of nonwoven fabric or suede while a polishing liquid containing a loose abrasive grain is continuously applied onto the polishing surface. The loose abrasive grain is composed of, for example, aluminum oxide, silicon oxide, cerium oxide, zirconium oxide, iron oxide, titanium oxide, manganese oxide or silicon carbide.
The conventional polishing process using a loose abrasive grain has a problem such that a polishing pad used has a very low modulus and thus the substrate material is not uniformly abraded over the entire surface to be polished, i.e., the corner portions of the material surface are excessively abraded upon polishing.
If a polishing pad is used together with a polishing liquid containing no loose abrasive grain, such as water having an adjusted pH value, the polishing power is too weak to complete the polishing within a reasonably short time. Even if a loose abrasive grain is Incorporated in an amount of about 10% or smaller in a polishing liquid, the polishing performance is still low. Usually 20 to 30% by weight or larger of a loose abrasive grain must be incorporated in a polishing liquid to obtain a polishing rate raised to a satisfying extent and to obtain a polished surface having no scratches or pits. However, the polishing liquid containing a loose abrasive grain must be continuously applied onto the polishing surface and thus the cost for loose abrasive grain is large. Further, a salient amount of a waste polishing liquid containing a loose abrasive grain is produced, and therefore, equipment for the waste disposal and the environmental pollution with the waste polishing liquid must be considered.
To solve the above-mentioned problems, a proposal has been made in Japanese Unexamined Patent Publication (hereinafter abbreviated to xe2x80x9cJP-Axe2x80x9d) No. H4-256581 wherein a synthetic abrasive stone comprising abrasive grain particles combined with a synthetic resin binder is used. It is described in this patent publication that the problem of non-uniform abrading can be mitigated or avoided. However, the use of a synthetic resin as a binder causes another problem such that the abrasive stone tends to be clogged with the synthetic resin, leading to reduction of polishing performance and productivity. Further, depending upon the abrading conditions, the synthetic resin occasionally causes contamination of a polished material with impurities from the synthetic resin.
An abrasive molding predominantly comprised of an abrasive silica grain is described in JP-A H10-264015. The following findings are described in this patent publication.
(1) The abrasive molding has a modulus higher than that of a polishing pad, and thus, excessive abrasion of the corner portions of the material surf ace occurring upon polishing can be minimized, and the substrate material can be uniformly abraded over the entire surface to be polished.
(2) The abrasive molding has a rough surface composed of silica particles, among which a multiplicity of pores are formed, and therefore, a problem such that an abrasive molding tends to be clogged during polishing can be minimized or avoided.
(3) The abrasive molding does not contain a synthetic resin and hence the abrasive molding exhibits high thermal resistance, chemical resistance and water resistance in the polishing process. Therefore, a high polishing efficiency can be obtained by using an appropriate polishing liquid in a temperature range reaching approximately the boiling point.
(4) The abrasive molding is composed of silica particles used as abrasive grain, and the molding does not contain a synthetic resin. Therefore, the abrasive molding does not cause contamination of a polished material.
(5) The smooth polished surface and the rate of polishing achieved with the abrasive molding are of the same level as or higher level than those of the conventional polishing processes using a polishing pad. The smooth finish and the rate of polishing are not decreased with a lapse of polishing time.
(6) The molding abrasive has a rough surface composed of silica particles. The hard and fine abrasive surface of the silica particles are brought into direct contact with a material to be polished, and hence, a polishing liquid which does not contain loose abrasive grains can be used for polishing with the abrasive molding.
(7) Even if an abrasive loose grain is used in combination with the abrasive molding, a high rate of polishing can be achieved with a polishing liquid containing the abrasive loose grain at a low concentration, as compared with the conventional polishing process using a polishing pad.
Although the abrasive molding composed of silica particles used as abrasive grain, described in JP-A H10-264015, is suitable for polishing machining process or chemicomechanical polishing process (hereinafter abbreviated to xe2x80x9cCMP processxe2x80x9d) for substrate materials such as a silicon wafer, oxide substrate, a chemical compound semiconductor substrate, a glass substrate and ceramic substrate, and optical materials, in the case when a mild substrate material is polished, full consideration must be given for selection of material of abrasive grain used and particle size thereof to completely avoid formation of worn marks caused by polishing. Even if a polishing liquid containing no abrasive grains is used, the abrasive molding sometimes causes worn marks on a polished surface and thus the optimal polishing conditions are difficult to determine.
A grinding stone consisting of sintered body of inorganic abrasive grains is described in JP-A H10-337669. It is taught in this patent publication that good results similar to those obtained by the abrasive molding of JP-A H10-264015, can be achieved by suitably selecting the material and particle size of abrasive grains, and the porosity and water absorption of the grinding stone. However, the polished surface of silicon wafer as an example of the material to be polished exhibits a surface roughness of approximately 3 nm as expressed in terms of center line mean surface roughness. The rate of polishing is not referred to in this patent publication.
In the above-stated JP-A H10-264015, the surface roughness of a polished material is expressed in terms of those values as measured by using a universal surface tester SE-3C available from Kosaka kenkyusho K. K. But, we found some difficulty in accurately measuring the surface roughness of a polished surface having a very low roughness by the same surface tester. Thus, we repeated the measurement of surface roughness of the polished surface obtained by the abrasive molding described in JP-A H10-264015, by using an atomic force microscope (AFM; xe2x80x9cSPI3600xe2x80x9d available from SII Co.), and found that the polished surface has a center line mean surface roughness of 0.6 nm to 1 nm, namely, the surface roughness is better than that of the polished surface obtained by the grinding stone described in JP-A H10-337669.
In view of the foregoing state of the prior art, it has been eagerly desired that to provide an abrasive molding which is capable of polishing a mild material at a high polishing rate to give a smooth polished surface having a high surface precision and no worn marks, and which is characterized, when the abrasive molding is used as an abrasive grain, in that determination of the material suitable for the particular substrate material and the particle size of abrasive grain can be easily made.
A primary object of the present invention is to provide an abrasive molding, which is suitable for polishing machining process or CMP process for substrate materials such as a semiconductor substrate, an oxide substrate and a glass substrate, and for optical materials for which a high precision machining is required, and further to provide a polishing disc provided with at least one of the abrasive molding.
More specifically, a primary object of the present invention is to provide an abrasive molding and a polishing disc provided with at least one abrasive molding; which abrasive molding is capable of polishing a material to be polished with a high efficiency by using a polishing liquid containing no loose abrasive grains or containing a minor amount of loose abrasive grains, and thus, the polishing cost is reduced and the problem of waste polishing liquid containing loose abrasive grains is mitigated; and is capable of polishing the material with a higher efficiency to give a smooth polished surface of the same level as or higher level than those of the conventional polishing processes using a polishing pad; said polished surface having no worn marks produced by polishing.
Another object of the present invention is to provide a abrasive molding which is especially suitable for polishing a mild material having a Vickers hardness not larger than 300 kg/mm2, and further to provide a polishing disc provided the abrasive molding.
In accordance with the present invention, there is provided an abrasive molding which comprises at least 90% by weight, based on the weight of the abrasive molding, of an inorganic material having a stock hardness of 50 kg/mm2 to 400 kg/mm2, and which has a relative density of 20% to 70% and an average particle diameter of 0.001 xcexcm to 50 xcexcm.